This invention relates to an AGC (Automatic Gain Control) circuit included in a TV (Television), a CATV converter, or the like, and more particularly to an AGC circuit having an AGC filter suitable for eliminating a hum noise included in an AGC signal.
With a CATV system or a TV, an AGC circuit is provided for controlling a level of a detected video signal in correspondence to changes of the intensity of a radio wave received by an antenna so as to keep the contrast of the reproduced video signal constant.
Generally, the AGC circuit is of a structure to prepare an AGC voltage from a detected video signal (composite video signal) to increase or decrease a gain of an RF (Radio Frequency) amplifier and a VIF (Video Intermediate Frequency) amplifier so that no change in the video level due to the intensity of a radio wave appears on a video signal. A mean value AGC circuit, a peak AGC circuit, and a keyed AGC circuit are well known as the AGC circuit.
In a conventional AGC circuit, an AGC filter (low pass filter) is generally used in order to eliminate a noise component in a high frequency band. In order to provide a desired characteristic of this AGC filter, various filter time constants are set according to transferring methods of the video signal.
An example of conventional AGC filters is shown in FIG. 1. In FIG. 1, reference numeral 100 denotes a VIF-IC, and a portion relating to the AGC circuit is illustrated. A VIF signal input from a VIF-IN terminal 10 is amplified by a VIF circuit 101, and is then video-detected at a video detection circuit 102. The detected video-signal is amplified at a video amplifier circuit 103, and is then delivered to a noise filter 104, at which excessively large noise components are eliminated. The noise eliminated video signal thus obtained is outputted from a VIDEO-OUT terminal 28. The video signal output from the VIDEO-OUT terminal 28 is input to a VIF AGC circuit 105, as an AGC control signal, through an aural carrier stop (trap) filter (not shown) and an AGC-IN terminal 26, and is then output from a VIF-AGC-OUT terminal 6. The AGC control voltage thus output is fed back to a VIF-AGC-IN terminal 7 through an AGC filter comprised of a resistor R and a capacitor C. The AGC voltage from which the noise-component in a high frequency band is eliminated by the AGC filter (R, C) is delivered to the VIF circuit 101. The amplification degree of the VIF circuit 101 is thus controlled in correspondence with the AGC voltage. Further, the AGC voltage is also delivered to a RF-AGC circuit 106. The RF-AGC circuit 106 outputs an AGC voltage (RF-AGC) from an RF-AGC OUT terminal 5 to control the amplification degree in a high frequency signal processing circuit.
As stated above, the AGC filter having the resistor R and the capacitor C is provided with the AGC circuit. In the conventional AGC circuit, the time constant of the AGC filter was fixed. However, in the case where the time constant is a fixed value, problems as described below would arise.
First, in the case where the value of the resistor R is fixed at a large value to set the filter time constant to a large value, there arises the problem that a hum component mixed in from the signal transmission system, etc. cannot effectively be eliminated, so the influence of the hum component appears on a television image. In the case of the CATV system, if a power supply voltage of a trank amplifier placed at the transmission system is weak in stability, an A.C. component of that power supply (i.e., a hum component) is mixed into a signal transmitted therethrough. As a result, such a hum component mixed into the transmitted signal would effect a hum-modulation on the RF signal to be transmitted. In a case where an RF signal is hum-modulated, the hum component is also superimposed on a detected video signal. Consequently, the hum component appears as any undesired fluctuation on a reproduced video image.
Further, the problem of the hum modulation also appears on the scramble processing used for protection from theft or cheating of a video. Particularly, in the case of the CATV system using an inverter type scramble processing, problems as described below would arise.
The inverter type scramble processing comprising the steps of inverting, by using a scrambler on the transmission center side, a video signal portion VS of an original TV signal (FIG. 2), applying an AM modulation to an RF carrier by the inverted waveform (FIG. 3), transmitting the signal thus modulated (FIG. 4), and inverting, by using a descrambler of a receiving side converter, the signal transmitted to thereby restore it to the original TV signal (FIG. 6). It is to be noted that, in the above respective figures, the portions on the left side represent a TV signal waveform, on one horizontal line (e.g. line 30), those on the right side represent a TV signal waveform on another horizontal line (e.g. line 150).
However, in the case where a hum component is superimposed on an inverted TV signal transmitted, the level (solid line) of an inverted TV signal varies with respect to the level (broken lines) of an original inverted TV signal as shown in FIG. 5. FIG. 5 shows the example where the hum component is superimposed by 2% on the inverted TV signal transmitted, wherein the level of the horizontal line 30 rises and the level of the horizontal line 150 lowers. As shown in FIG. 6, in the case where the inverted TV signal whose level is changed by the hum component is descrambled and only an image signal portion is re-inverted to restore the original TV signal, the magnitude of the TV signal of the horizontal line 30 becomes small, while the magnitude of the TV signal of the horizontal line 150 becomes large. Thus, a difference B of luminance signal level and a difference S of horizontal synchronizing signal level are produced.
FIG. 7 shows a waveform of a video signal detected by a conventional converter or a video signal obtained by remodulating the waveform shown in FIG. 6 by a CATV converter and then detecting it by TV. That is, since the time constant of AGC filter of a television is small, the signal levels are fixed to allow the levels of the respective synchronizing signal portions to be a predetermined constant value. Accordingly, the level of the video signal portion VS is lowered some degree at the horizontal line 30 and is raised at the horizontal line 150. As a result, at the signal portions on which the hum component is superimposed, luminance levels (i.e., D.C. levels) might fluctuate in above-described manner, resulting in deteriorated picture quality.
In addition, the problem of the hum modulation occurs in the case where the hum elimination percentage or rate is low at a control voltage for tuning control. In a TV or a CATV converter, in order to eliminate the noise component at the sync chip portion and/or the pedestal portion of a horizontal synchronizing signal of a detected video output signal, a technique is employed to replace those portions by a fixed D.C. voltage (U.S. Pat. No. 5,113,439).
In such a case, since the influence of the hum modulation is left only at the image signal portion, there also takes place the problem that thick and thin portions of the scanning line appear on the image reproduced.
Second problem is as follows. In the case where the value of the resistor R is set to a small value to thereby set the filter time constant to a small value, while the hum-modulated portion of an RF signal is eliminated from the detected video signal, the noise component would be superimposed also on the video signal portion in proportion to changes in the noise component on the sync chip portion or the pedestal level. As a result, fluctuation of an image resulting from the noise component appears on the reproduced image.
Accordingly, when the time constant of the AGC filter is fixed as described above, there was the inconvenience that the noise component cannot be completely eliminated.